Marine biogenic emissions of benzene and toluene and their contribution to secondary organic aerosols over the polar oceans

Reactive trace gas emissions from the polar oceans are poorly characterized, even though their effects on atmospheric chemistry and aerosol formation are crucial for assessing current and preindustrial aerosol forcing on climate. Here, we present seawater and atmospheric measurements of benzene and toluene, two gases typically associated with pollution, in the remote Southern Ocean and the Arctic marginal ice zone. Their distribution suggests a marine biogenic source. Calculated emission fluxes were 0.023 ± 0.030 (benzene) and 0.039 ± 0.036 (toluene) and 0.023 ± 0.028 (benzene) and 0.034 ± 0.041 (toluene) μmol m−2 day−1 for the Southern Ocean and the Arctic, respectively. Including these average emissions in a chemistry-climate model increased secondary organic aerosol mass concentrations only by 0.1% over the Arctic but by 7.7% over the Southern Ocean, with transient episodes of up to 77.3%. Climate models should consider the hitherto overlooked emissions of benzene and toluene from the polar oceans.


Supplementary Text
Details on the Chl a data from both cruises Chl a data from both cruises in depth profiles and underway are from fluorescence sensors. The underway Chl a data from the Antarctic cruise are relatively uncertain due to large sensor drift, but the data has been corrected using the sensor mounted on the CTD frame. For the Arctic cruise, Chl a measured from CTD frame and underway agree within 0.1 mg m -3 . Thus, our Chl a data does not explicitly account for quenching of fluorescence and is not proportional to phytoplankton biomass. It is of value to compare Chl a with benzene and toluene distributions in depth profiles and underway data to obtain information on the possible biological source, though it is not possible to derive from this how benzene and toluene concentrations correlate with phytoplankton biomass.
Details on the computation of benzene and toluene seawater and ambient air concentrations as well as fluxes During the Antarctic cruise, the PTR-MS was calibrated weekly using a certified gas standard (Apel-Riemer Inc.). During the Arctic cruise, benzene was calibrated using a certified gas standard during installation before the campaign. A post-cruise calibration with a certified gas standard was applied to toluene. These calibration curves were used to calculate equilibrator headspace gas phase mole fractions and ambient air mole fractions. During the Arctic cruise, this gas calibration displayed a humidity dependence for benzene and toluene, which was not observed during the Antarctic deployment due to the higher drift tube voltage.
To calculate ambient air measurements, hourly measurement of outside air scrubbed by a 450°C Pt-catalyst is used as a blank for toluene. Daily measurements of zero air carrier gas are used as a blank for benzene. Measurement of zero air from a gas canister are used as a blank for benzene because it seemed that during episodes of high benzene air mole fractions from sampling ship exhaust the Pt-catalyst does not fully remove benzene. For most of the deployment, the Pt-catalyst agrees with the zero air. For both cruises, measurements at 5m from CTD casts and proximate underway measurements agree very well suggesting no contamination from either sampling technique. In Wohl et al. (60), we discussed that average concentrations of toluene measured from the underway intake system were slightly higher than those measured from the CTD casts. In this manuscript, we display measurements from both sampling techniques which show no obvious bias. It is possible that mean underway concentrations were slightly higher as they by chance captured episodes of higher concentrations. We note that for the Antarctic cruise, benzene and toluene seawater measurements did not appear to be affected by photochemical production within the SFCE.
The measurement noise (Table S1) is calculated as the standard deviation of the residual of the interpolation of the measurement background (as laid out in Wohl et al. (46)). The limit of detection is defined as three times the measurement noise. Measurement noise was calculated to be 8 and 10 pmol mol -1 for benzene and toluene in the air. The measurement noise in seawater for the Southern Ocean cruise was 1 and 4 pmol dm -3 for benzene and toluene while it was 5 and 10 pmol dm -3 for benzene and toluene during the Arctic cruise.
The seawater measurement noise is slightly higher in the Arctic compared to the Southern Ocean cruise due to less frequent blanks and different PTR-MS quadrupole data collection settings.
Measurement noise and LOD are low enough to detect these gases in seawater while air mole fractions were often close to the LOD. Location of the CTD stations is indicated by a filled circle coloured by the order of sampling. Hollow squares and date labels (DD/MM/YYY) are used to give an indication of the sampling date. Interruptions in the cruise track and underway auxiliary data (B) are due to interruptions in the ship underway logging system (79). All the map data were created from public domain GIS data found on the Natural Earth website (http://www.naturalearthdata.com, last access: 15 April 2021). They were read into Igor using the Igor GIS XOP beta. The sea-ice-covered area during the Arctic cruise (B) is approximately indicated for illustration purposes as a shaded area due to the dynamic nature of sea ice and difficulties in conveying this information for a month-long deployment. The approximate location of the sea ice edge is based on the average sea ice cover for the whole cruise duration using AMSR2 satellite data.
The cruise sampling tracks of the two cruises in the polar oceans are presented in Figure S1    We test for diurnal variability in the measured seawater concentrations from the Southern Ocean by using the local solar time to remove the influence of the ship track crossing multiple time zones. The measured seawater concentrations were binned in 24 hourly bins and the standard deviation and standard error was calculated for each bin ( Figure S2). Figure S2 shows that measurements of benzene and toluene during solar zenith are generally a little higher than the other measurements, but there is no substantial diurnal variability in seawater benzene and toluene concentrations and this variability is not statistically significant. The Arctic data was not tested for diurnal variability due to 24 h light during the sampling period and the relatively smaller dumber of data points compared to the Southern Ocean cruise.      Table S1 shows that using higher air mole fractions in the Arctic does not change the conclusion of oceanic outgassing of benzene and toluene in the Arctic. Using higher air mole fractions, decreases the estimated benzene saturation by 74 % and the flux by 52 %. Similarly, using higher toluene mole fractions decreases the saturation by 87 % and the flux by 26 %. This calculation gives an appreciation of the potential uncertainty of the fluxes reported for the Arctic deployment due to the choice in air mole fraction.  Antarctic, <-55ºS: 0.0236 μmol m -2 d -1 Arctic, >60ºN: 0.0341 μmol m -2 d -1 Antarctic, <-55ºS: 0.0390 μmol m -2 d -1 maxBT Arctic, >60ºN: 0.158 μmol m -2 d -1 Antarctic, <-55ºS: 0.358 μmol m -2 d -1 Arctic, >60ºN: 0.268 μmol m -2 d -1 Antarctic, <-55ºS: 0.158 μmol m -2 d -1